Mini applications that allow advanced communication features used in social network sites like MySpace or Facebook might not be a security risk by means of vulnerability (viruses, trojans, phishing etc.) but they might corrupt user's privacy. In Facebook it is common practice to get access to a mini application only after checking a checkbox that tells the app's developer "who I am and [who is allowed to] access my information", a similar practice can be found in MySpace, where developers get access to a person's profile data.

Even if developers won't do any harmful things with these data (some of them use it for tweaking their advertising, though), the sheer masses of user data will make some of them greedy. It's just too easy to build a scraper that crawls for user data that give deep insight in a user's personal interests. The end of this development is identity theft. (Source)